Apparatus and method for sensing force on a robotically controlled medical instrument

ABSTRACT

A medical system comprises a medical probe including an elongated probe body, a lumen extending within the probe body, an axially flexible section, and a push-pull member slidably disposed within the lumen. The system comprises a ditherer mechanically coupled to the member for cyclically displacing it axially back and forth within the lumen, such that the ends of the probe body are axially displaced relative to each other via the axially flexible section. The system further comprises a sensor for sensing a force axially applied to the distal end of the probe body. A method comprises introducing a medical probe into a patient, axially dithering the distal end of the medical probe back and forth relative to the proximal end of the medical probe, and sensing a force applied between tissue of the patient and the distal end of the medical probe while the distal end is axially dithered.

RELATED APPLICATION DATA

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 12/269,684, filed Nov. 12, 2008, which isincorporated by reference into the present application in its entirety.

FIELD OF INVENTION

The inventions disclosed herein relate generally to minimally-invasivemedical instruments and systems, such as manually or roboticallysteerable catheter systems, and more particularly to steerable cathetersystems having tissue contact force sensing capability, and their usefor performing minimally invasive diagnostic and therapeutic proceduresin a human or other animal body.

BACKGROUND

Minimally invasive procedures are preferred over conventional techniqueswherein the patient's body cavity is opened to permit the surgeon'shands access to internal organs. Thus, there is a need for highlycontrollable, yet minimally sized, medical instruments to facilitateimaging, diagnosis and treatment of tissues which may lie deep within apatient, and which may be accessed via naturally-occurring pathways,such as blood vessels, other body passages or lumens, viasurgically-created wounds of minimized size, or through combinationsthereof.

Currently known minimally invasive procedures for the treatment ofcardiac and other disease conditions employ both manually androbotically controlled instruments, such as steerable catheters, whichmay be inserted transcutaneously into body spaces such as the thorax orperitoneum, transcutaneously or percutaneously into lumens such as theblood vessels, through natural orifices and/or lumens such as the mouthand/or upper gastrointestinal tract, etc. Such devices are well suitedfor performing a variety of minimally invasive diagnostic andtherapeutic procedures.

When manually controlling an elongate instrument, such as a steerablecatheter having a proximal end handle, the physician operator (whilegrasping the handle) can axially push on the proximal end of thecatheter and attempt to tactilely “feel” the catheter distal end makecontact with pertinent tissue structures located deep in the patient'sbody, such as the walls of the heart. Some experienced physiciansattempt to mentally determine or gauge the approximate force beingapplied to the distal end of a catheter due to contact with tissuestructures or other objects, by interpreting the loads they tactilelysense at the proximal end of the inserted catheter with their fingersand/or hands. Such an estimation of the force, however, is quitechallenging given the generally compliant nature of manyminimally-invasive instruments, associated frictional loads, dynamicpositioning of the instrument versus nearby tissue structures, and otherfactors.

Robotically controlled catheters have a proximal interface coupled withan instrument driver comprising, for example, one or more motors thatare selectively actuated to induce intra-body navigation of the distalportion of the catheter in response to commands input by an operator ata master input station or using some other device that may be locatedremotely from the patient. Thus, even the most gifted of operatorphysicians would be unable to gauge forces applied to the distal end ofthe catheter through tactile feel at the proximal end.

Regardless of the manual or electromechanical nature of the drivingmechanism for a diagnostic or interventional catheter, the operatorperforming the procedure would prefer to have accurate, timelyinformation regarding the forces experienced at the distal portion ofthe catheter, such as loads applied by or to the catheter from adjacenttissues and other objects.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the disclosed inventions, a medicalinstrument system comprises a medical probe, a dithering mechanism fixedrelative to the probe (e.g., mounted on or in a handle of the probe, oron an instrument driver to which the probe may be operatively coupled),wherein the dithering mechanism mechanically actuates a push-pull memberextending axially through the probe for causing a distal tip of theprobe to dither (i.e., move back and forth) axially relative to a moreproximal portion of the probe, and at least one force sensor located ata proximal end portion of the push-pull member and configured to senseforces applied on or by (collectively “to”) the push-pull member and,thus, the distal tip of the probe.

More particularly, the medical probe has an elongated probe body (e.g.,a flexible intravascular catheter body) having a distal tip section, abendable section located proximally to the distal tip section, and anaxially variable-length (e.g., an axially compressible or translatable)section interposed between the bendable and distal tip sections. In oneembodiment, the axially variable-length section of the probe bodycomprises an axially compressible polymer sleeve. In another embodiment,the axially variable-length section of the probe body comprises abellows. In still another embodiment, the axially variable-lengthsection of the probe body comprises at least one seal that allows one ofthe bendable and distal tip sections of the probe body to slide axiallywithin the other. The medical probe may further include one or moreoperative elements, such as tissue sensing, pacing and/or ablationelectrodes, mounted to the distal tip section.

One or more steering control elements (e.g., wires) extend through theprobe body for deflecting the distal tip section in at least onedirection. The distal ends of the control element(s) are preferablyaffixed to the probe body at a location proximal of the axiallyvariable-length section. In one exemplary embodiment, the probe body hasfour control elements circumferentially spaced 90 degrees apart andextending from a proximal end portion of the probe body to a steeringanchor ring fixed at a distal end of the bendable section. If the probeis manually controlled, the proximal end of the probe body will includea handle having one or more steering mechanisms therein for manipulatingthe control elements in order to controllably deflect the bendablesection of the probe. If the probe is robotically controlled, thecontrol elements are coupled with respective motors in an instrumentdriver to which the probe is operatively mounted in order tocontrollably deflect the bendable section of the probe.

The push-pull member may comprise one or more relatively stiff wires,and is slidably disposed through an axially extending lumen of theprobe, with a distal end of the push-pull member being affixed to thedistal tip section of the probe body, and with a proximal end of thepush-pull member extending out of an opening in the proximal end of theprobe body for coupling to the dithering mechanism. In particular, thepush-pull member (and, thus, the distal tip section of the probe body)is axially displaced by the dithering mechanism relative to therespective handle or instrument driver, and also relative to thesteering control elements which terminate in the probe body proximallyof the axially variable-length section.

In one embodiment, the medical probe further comprises a substantiallyincompressible stiffening coil that extends axially through the innerlumen of the probe, the push-pull member extending through a center of,and being translatable relative to, the stiffening coil, with thecoefficient of friction between the outer surface of the stiffening coiland the inner surface of the probe body defining the lumen beingminimized. In one embodiment, the stiffening coil is affixed to theprobe body at a steering wire anchor member located at a distal end ofthe bendable section. In another embodiment, a distal end portion of thestiffening coil extends past the steering wire anchor member and throughthe axially variable-length section before terminating against thedistal tip section. The pitch of the individual coil windings in thisdistal portion of the stiffening coil is significantly opened, and thediameter of the individual windings is substantially expanded, so thatthe coil portion extending past the anchor member functions as aresilient tensioning spring to help maintain the push-pull member intension as the push-pull member is dithered back and forth through theprobe body lumen.

The dithering mechanism mechanically couples with (e.g., “grasps”) aproximal end portion of the push-pull member extending out of a proximalend of the probe body, and is configured for cyclically displacing thepush-pull member axially back and forth within the lumen of the probebody, such that the respective bendable and distal tip sections of theprobe body are axially displaced relative to each other via the axiallyvariable-length section disposed therebetween. One or more force (orload) sensors are mounted to the dithering mechanism or at some otherlocation that is fixed with respect to the motion of the push-pullmember. In particular, the force sensor(s) are arranged and configuredfor measuring an axial force (or “load”) on the push-pull member, whichincludes any external force axially applied to the distal tip of theprobe body.

The medical instrument system further comprises a computer (moregenerically, a “processor”) programmed for obtaining a “baseline” forcemeasurement based on signals received from the force sensor(s) when thepush-pull member is dithered back and forth without any external axialforce applied to the distal tip of the probe body (a conditiondetermined by the system operator, e.g., during a system initiationprocess). Thereafter, a “total” force measurement is obtained based onsignals received from the force sensor(s) when the push-pull member isdithered back and forth when external axial forces are being applied tothe distal tip of the probe body. The net external axial force appliedto the distal tip of the probe body is then calculated by the processorby subtracting the baseline force measurement from the total forcemeasurement.

In one embodiment, the axially variable-length section of the probe bodyis a compressible polymer sleeve with a spring member axially disposedtherein to maintain tension on the push-pull member during the ditheringoperation. In particular, a proximal end of the spring is compressibleagainst a distal-facing surface of a steering element anchor ring thatalso defines a distal end of the bendable section, and a distal end ofthe spring is compressible against a proximal-facing surface of theprobe distal tip section.

Prior to attachment of the push-pull member to the dithering mechanism,the tensioning spring is preferably in an unloaded state. Wheninitiating the instrument system for performing a procedure in apatient, the proximal end portion push pull member is retractedproximally relative to the distal tip section of the probe body, therebyretracting the distal tip section relative to the bendable section, andcompressing the spring against the anchor ring. Once a specified load onthe push-pull member (as measured by the force sensor(s)) is reached,the proximal end portion of the push-pull member is affixed to thedithering mechanism, and dithering of the push-pull member is commenced,with a back-and-forth stroke distance traveled by the push-pull membergoverned by maintaining the measured (or “sensed”) load on the push-pullmember within a specified operating range.

As the dithering mechanism “pushes” the push-pull member (and, thus, thedistal tip section of the probe) in a distal direction axially relativeto the handle or instrument driver on which the dithering mechanism ismounted, the spring decompresses, thereby decreasing the measured loadon the push-pull member until the lower end of the operating load rangeis reached, which is preferably at a point when the spring returns to aslightly pre-loaded state but without allowing the spring to beingaxially tensioned. Preferably, there is always at least some pre-load inthe spring, i.e., the spring is not allowed to return to a fullyunloaded position. It is also preferable that the spring is never placedin a fully compressed condition.

Thereafter, the dithering mechanism reverses direction and “pulls” thepush-pull member (and, thus, the distal tip section of the probe) in aproximal direction axially relative to the handle or instrument driveron which the dithering mechanism is mounted, and the spring is againcompressed (but not fully) against the anchor ring, resisting the inwardmotion of the probe distal tip and increasing the measured load on thepush-pull member until the upper end of the load range is reached. Thedithering cycle is then repeated.

In some embodiments, the medical probe may comprise a fluid deliverytube extending axially through the inner probe lumen for deliveringcooling fluid, e.g., saline from a fluid supply port located at aproximal end of the probe to the distal tip of the probe. The coolingfluid may be circulated within an interior region of the tip andreturned to the proximal end of the probe (and out a fluid outlet port)through an additional fluid return tube. Alternatively, the fluid may bereleased into the patient's body through one or more fluid outletpassages formed in the probe distal tip. Either way, in accordance withcertain embodiments of the disclosed inventions, the fluid delivery tubemay also be used as the push-pull member for dithering the distal tipsection of the probe relative to the bendable section. In suchembodiments, a proximal portion of the fluid delivery tube extending outof the proximal end of the probe is mechanically coupled to thedithering mechanism in the same manner as the above-described push-pullmember.

In accordance with another aspect of the present inventions, a medicalmethod is provided, the method comprising introducing a medical probehaving an elongated probe body into a patient (e.g., intravascularly),axially dithering the distal end of the probe body back and forthrelative to a more proximal portion of the probe body, and sensing aforce applied between tissue of the patient and the distal end of theprobe body while the distal end of the probe body is being axiallydithered. In embodiments of the method, the force applied between thetissue and the distal end of the probe body is determined by firstobtaining a baseline force measurement when the distal end of the probebody is axially dithered back and forth without an external axial forceaxially applied between the tissue and the distal end of the probe body,subsequently obtaining a total force measurement when the distal end ofthe probe body is dithered at the same time an external axial force isaxially applied between the tissue and the distal end of the probe body,and then subtracting the baseline force measurement from the total forcemeasurement.

Other and further aspects and features of the disclosed inventions willbe evident from reading the following detailed description of thepreferred embodiments, which are intended to illustrate, not limit, thedisclosed inventions.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of various embodiments ofthe disclosed inventions, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the disclosedinventions are obtained, a more particular description of the disclosedinventions described above is provided herein by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only some of the possibleembodiments of the invention and are not therefore to be consideredlimiting of its scope, the disclosed inventions are described andexplained herein with additional specificity and detail through the useof the accompanying drawings, in which:

FIG. 1 is a perspective view of a robotically controlled medicalinstrument system constructed in accordance with one embodiment of thedisclosed inventions;

FIG. 2 is a perspective view of a robotically controlled catheterinstrument assembly that may be employed in the medical robotic systemof FIG. 1;

FIG. 3 is a top view of the robotic catheter assembly of FIG. 2;

FIG. 4 is a cross-sectional view of a respective introducer sheath andworking catheter used in the robotic catheter assembly of FIG. 2,particularly taken along the line 4-4;

FIG. 5 is a cross-sectional view of the working catheter used in therobotic catheter assembly of FIG. 2, particularly taken along the line4-4;

FIG. 6 is a plan view of the distal end of the working catheter used inthe robotic catheter assembly of FIG. 2;

FIG. 7 is a partially cut-away view of the distal end of the workingcatheter used in the robotic catheter assembly of FIG. 2;

FIG. 8 is a conceptual view of a force sensing assembly used in therobotic catheter assembly of FIG. 2;

FIGS. 9 a-d illustrates a method of operating the robotic cathetersystem of FIG. 1 to sense a force applied between endocardial tissue andthe distal end of the working catheter illustrated in FIG. 6;

FIG. 10 is a plan view of the distal end of an alternate workingcatheter that may be used in the robotic catheter assembly of FIG. 2;

FIG. 11 illustrates a method of operating the robotic catheter system ofFIG. 1 to sense a force applied between the inner lining of a stomachand the distal end of the working catheter illustrated in FIG. 10;

FIG. 12 is a perspective view of the distal end of still another workingcatheter that may be used in the robotic catheter assembly of FIG. 2;

FIG. 13 is a perspective view of the distal end of yet another workingcatheter that may be used in the robotic catheter assembly of FIG. 2;

FIG. 14 is a partially cut-away perspective view of the distal endportion of an alternate embodiment of a working catheter that may beused in the robotic catheter assembly of FIG. 2, in which a flexiblepolymer sleeve is used instead of a bellows for the compressiblesection, with a spring incorporated into the compressible polymer sleevefor tensioning the push-pull dithering member;

FIGS. 15 a and 15 b are partially cut-away perspective views of therespective distal and proximal end portions of still another embodimentof a working catheter that may be used in the robotic catheter assemblyof FIG. 2, in which a fluid supply tube used to supply cooling fluid tothe distal tip of the catheter is also used as a push-pull member fordithering the distal tip of the catheter, and in which a distal endportion of an axial stiffening coil formed as a spring polymer sleevesection;

FIG. 16 is a side view of the catheter as it is bent in one direction;

FIG. 17 depicts a model of the forces applied to a catheter as it isarticulated which treats the catheter and each tendon member as springswith different spring constants depending on their stiffness; and

FIG. 18 is a graph of displacement of the guide tendon vs. the overallstrain (or change in length) of the shaft.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Before describing the specific illustrated embodiments in detail, it isto be understood that, unless otherwise indicated, the inventionsdisclosed herein need not be limited to applications in humans. As oneof ordinary skill in the art would appreciate, variations of theinvention may be applied to other mammals as well. Moreover, it shouldbe understood that embodiments of the disclosed inventions may beapplied in combination with various catheters, introducers, or othersurgical tools for performing minimally invasive surgical procedures.

As used in this specification and the appended claims, the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, the term “a member” isintended to mean a single member or a combination of joined or separatemembers, “a fluid” is intended to mean one or more fluids, or a mixturethereof. Furthermore, the words “proximal” and “distal” refer todirection close to and away from, respectively, an operator (e.g.,surgeon, physician, nurse, technician, etc.) who would insert themedical device into the patient, with the tip-end (i.e., distal end) ofthe device inserted inside a patient's body first. Thus, for example,the catheter end inserted inside the patient's body would be the distalend of the catheter, while the catheter end left outside the patient'sbody would be the proximal end of the catheter.

Before describing an embodiment of a robotic catheter system constructedin accordance with the disclosed inventions, it may be helpful to firstdescribe one robotic catheter system that has been previously designedto sense force. U.S. patent application Ser. No. 11/678,001 (the '001application), which is commonly assigned with the present application,discloses robotically-navigated interventional systems and methodshaving the capability to sense force between a distal end of a workinginstrument (such as a catheter) and the surface of a body cavity orlumen (referred to collectively as a “body space”). The robotic systemof the '001 application not only detects contact between the workinginstrument and the surface, but also measures the magnitude of theforce, also called the load. Such systems and methods can also be usedto detect contact with tissue structures.

In particular, the robotic system described in the '001 applicationcomprises a robotic instrument driver that directly interfaces with acoaxial arrangement of an introducer sheath and a guide catheter, and anoperator control station for remotely controlling movements of theintroducer sheath and guide catheter through the instrument driver. Aworking instrument, e.g., a cardiac mapping or ablation catheter whichcan be manually operated by a physician, is disposed through therobotically controlled guide catheter via a hemostatic (e.g., Touhy)valve. The guide catheter, in turn, is disposed through the roboticallycontrolled introducer sheath. The instrument driver comprises separateinstrument interfaces for coupling to respective instrument driveradapters (sometimes referred to as “splayers”) attached at proximal baseportions of the sheath and catheter in a manner providing for controlledmanipulation of respective steering control elements (wires) extendingthrough the sheath and catheter to provide them with independentsteering actuation. The instrument driver may also move the instrumentinterfaces and, thus, the respective introducer sheath and guidecatheter splayers relative to each other to provide independent axialinsertion or retraction movements to the introducer sheath and guidecatheter along a longitudinal axis.

The robotic system described in the '001 application measures a forceapplied on a distal end of the working instrument using a ditheringtechnique. In particular, the working instrument is “dithered” withrespect to the guide catheter through which it extends by moving (i.e.,axially translating) the working instrument back and forth relative tothe guide catheter in a repeated cyclic motion to overcome frictionalchallenges that would otherwise complicate measuring loads at theinstrument's distal end. That is, if the working instrument ispositioned down the lumen of the guide catheter so that the distal endof the working instrument extends out slightly beyond (and out of) theopen distal end of the guide catheter, it may be difficult to accuratelysense forces applied to the distal end of the working instrument due tothe frictional complications of the physical relationship between theworking instrument and guide catheter. In a steady state wherein thereis little or no relative axial or rotational motion between the workinginstrument and the guide catheter, the static coefficient of friction isapplicable and, as such, there are relatively large frictional forceskeeping the working instrument in place relative to the guide cathetersuch that there is no relative movement between the two when measured atthe proximal end of the working instrument after the various frictionallosses are absorbed along its length. To release this relatively tightcoupling and facilitate an accurate measurement of forces applied to thedistal end of the working instrument, the dithering motion is used toeffectively break loose this frictional coupling with the guidecatheter.

The dithering motion is provided by a “ditherer” (more generically,“dithering mechanism”) that is mechanically coupled to the proximal endportion of the working instrument extending out from the proximal end ofthe guide catheter. A bellows is provided on the hemostatic valvethrough which the working instrument is inserted into the guide catheterto allow for dithering of the working instrument, while preventingfluid(s) from exiting the guide catheter. One or more sensors areprovided on (or adjacent to) the ditherer for measuring forces appliedat the distal end of the working instrument and transmitted through itsshaft to the ditherer. The sensor(s) are operatively coupled to acontroller that determines a baseline dynamic frictional force betweenthe working instrument and guide catheter based on output signals fromthe sensor(s) that represent the respective insertion and withdrawalforces generated by the moving working instrument relative to the guidecatheter at a time when no external forces are exerted on the workinginstrument. Thus, any additional forces applied to the distal end of theworking instrument can be calculated by the controller by subtractingthe baseline frictional force from the total measured force.

Thus, the specific dithering techniques taught and discussed in the '001application provides an accurate technique for sensing forces applied tothe distal end of a manually operated working instrument extendingthrough (and out the distal end of) a robotically controlled guidecatheter. The contents of the '001 application are incorporated hereinby reference. The embodiments disclosed herein are directed to the useof dithering systems and techniques for sensing forces applied to thedistal end of an “all-in-one” instrument that may be a manuallycontrolled instrument having a proximal handle, or a roboticallycontrolled instrument (e.g., a robotically steerable catheter having atherapeutic and/or diagnostic function), despite the fact that theproximal end of the instrument is affixed to a robotically controlledinstrument driver.

Referring to FIG. 1, one embodiment of a robotically controlled cathetersystem 10 constructed in accordance with the disclosed inventions willnow be described. The system 10 generally comprises an operating table12 having a movable support-arm assembly 14, an operator control station16 located remotely from the operating table 12, and a robotic catheterassembly 18 mounted to the support-arm assembly 14 above the operatingtable 12. In addition to the systems disclosed in the above-incorporated'001 application, exemplary robotic catheter systems that may bemodified for constructing and using embodiments of the inventionsdisclosed herein are described in detail in the following U.S. patentapplications, which are all expressly incorporated herein by referencein their entirety: U.S. patent application Ser. No. 11/073,363, filedMar. 4, 2005; U.S. patent application Ser. No. 11/179,007, filed Jul. 6,2005; U.S. patent application Ser. No. 11/418,398, filed May 3, 2006;U.S. patent application Ser. No. 11/481,433, filed Jul. 3, 2006; U.S.patent application Ser. No. 11/637,951, filed Dec. 11, 2006; U.S. patentapplication Ser. No. 11/640,099, filed Dec. 14, 2006; U.S. PatentApplication Ser. No. 60/833,624, filed Jul. 26, 2006; and U.S. PatentApplication Ser. No. 60/835,592, filed Aug. 3, 2006.

The control station 16 comprises a user interface 20 that is operativelyconnected to the robotic catheter assembly 18. A physician or otheroperator 22 may interact with the user interface 20 to operate therobotic catheter assembly 18. The user interface 20 is connected to therobotic catheter assembly 18 via a cable 24 or the like, therebyproviding one or more communication links capable of transferringsignals between the control station 16 and the robotic catheter assembly18. Alternatively, the user interface 20 may be located in ageographically remote location and communication is accomplished, atleast in part, over a wide area network such as the Internet. The userinterface 20 may also be connected to the robotic catheter assembly 18via a local area network or even wireless network that is not located ata geographically remote location.

The control station 16 also comprises one or more monitors 26 used todisplay various aspects of the robotic instrument system 10. Forexample, one or more images of the introducer sheath and workingcatheter of the robotic catheter assembly 18 (described in furtherdetail below) may be displayed in real time on the monitors 26 toprovide the physician 22 with the current orientation of the variousdevices as they are positioned, for example, within a body lumen orregion of interest. The control station 16 further comprises a computer28, which may comprise a personal computer or other type of computerwork station, for performing the data processing operations disclosedherein.

The support-arm assembly 14 is configured for movably supporting therobotic catheter assembly 18 above the operating table 12 to provideconvenient access to the desired portions of the patient (not shown),and also to provide a means to lock the catheter assembly 18 intoposition subsequent to the preferred placement. In one embodiment, thesupport-arm assembly 14 comprises a series of rigid links 30 coupled byelectronically braked joints 32, which prevent joint motion whenunpowered, and allow joint motion when energized by the control station16. In an alternative embodiment, the rigid links 30 may be coupled bymore conventional mechanically lockable joints, which may be locked andunlocked manually using, for example, locking pins, screws, or clamps.The rigid links 30 preferably comprise a light but strong material, suchas high-gage aluminum, shaped to withstand the stresses and strainsassociated with precisely maintaining three-dimensional position of theweight of the catheter assembly 18.

Referring further to FIGS. 2 and 3, the robotic catheter assembly 18will now be described in greater detail. The catheter assembly 18generally comprises a remotely controlled instrument driver 34, with arobotic introducer sheath 36 and a robotically controlled workingcatheter 38 mounted to the instrument driver 34 in a coaxialrelationship, i.e., with the working catheter 38 extending coaxiallythrough a lumen of the introducer sheath 36. A dithering force sensingassembly 40 is mounted to the instrument driver 16 in mechanicalcommunication with a push-pull member 106 (e.g., comprising one or morewires) extending from the open proximal end of the working catheter 38,as described in greater detail herein. The robotic catheter assembly 18may also include a sterile drape (not shown) that covers the instrumentdriver 34.

Referring also to FIG. 4, the introducer sheath 36 comprises anelongated sheath body 42 having a proximal end 44, a distal end 46, anda working lumen 48 extending through the sheath body 42 between theproximal and distal ends 44, 46. It will be appreciated that thegeometry and size of the sheath working lumen 48 is selected inaccordance with the corresponding cross-sectional geometry and size ofthe working catheter 38. The sheath body 42 preferably has alow-friction inner surface or layer 50, e.g., a coating of silicone orpolytetrafluoroethylene, to provide a low-friction surface toaccommodate movement of the working catheter 38 within the working lumen48, with a stiffening layer 52 (e.g., a braided material or a metallicspine) disposed over the outer surface of the inner layer 50, and anouter polymer jacket 54 disposed over the outer surface of thestiffening layer 52.

The introducer sheath 36 further comprises a control element in the formof steering wire 56 extending through a steering lumen 58 disposedwithin the wall of the sheath body 42, and in particular, extendingthrough the polymer jacket 54. The distal end of the steering wire 56 issuitably mounted to an anchoring element, e.g., an annular ring (notshown), located at the distal end 46 of the sheath body 42, and theproximal end of the steering wire 56 extends out the proximal end 44 ofthe sheath body 42, so that it can be suitably coupled to instrumentdriver 34, via an introducer sheath splayer 112, as discussed below.Thus, it can be appreciated that the distal end 44 of the sheath body 42can be alternately deflected and straightened via actuation of thesteering wire 56 by a motor in the instrument driver 34 coupled to thesteering wire via the sheath splayer 112. In alternate embodiments, theintroducer sheath 36 may be provided with one or more additionalsteering wires controlled by respective motors in the instrument driver34, via the sheath splayer 112, to provide for increased distal bendingcapability.

The working catheter 38 is an exemplary embodiment of an elongate probeinstrument of the presently disclosed inventions, and may take the formof any number of types of catheters. In the illustrated embodiment, theworking catheter 38 takes the form of an electrophysiology/ablationcatheter, wherein forces imposed between the distal end of the workingcatheter 38 can be communicated to an ablation energy generator (notshown) as an input parameter. For example, the ablation energy generatormay be prevented from operation unless the sensed force on the distalend of the working catheter is within a range that confirms there isadequate tissue contact required to provide effective treatment. By wayof another example (not mutually exclusive of the prior example), theablation energy generator may automatically compute and set andparticular ablation power, ablation time, temperature, etc., as afunction of the sensed force at the distal end of the working catheter.It should be noted that the working catheter 38 may alternatively oradditionally carry other types of operative elements, such as a tissuemanipulation tool or other device, also called “end effectors”, such ase.g., an imaging device or cutting tool disposed on the distal end ofthe catheter 38. It should also be noted that, although the workingcatheter 38 is described as an intravascular catheter, other types ofmedical probes may be used. For example, the working catheter 38 maytake the form of an endoscopic surgical instrument or other elongatedmedical instrument. Depending on the application, the working catheter38 can be rigid or semi-rigid.

The working catheter 38 passes through the lumen 48 of the introducersheath 36, and is axially and rotatably moveable relative thereto. Asshown in FIGS. 2 and 3, the working catheter 38 may project distally outthe open distal end 44 of the introducer sheath body 42, although theworking catheter 38 may also be withdrawn proximally such that itsdistal end is substantially flush with the distal end 44 of the sheathbody 42, or withdrawn even further such that its distal end is disposedcompletely within the sheath body 42. The working catheter 38 may bemovably positioned within the working lumen 48 of the introducer sheath36 to enable relative insertion of the two devices, relative rotation(or “roll”) of the two devices, and relative steering or bending of thetwo devices, particularly when the distal end of the working catheter 38is inserted into a patient's body beyond the open distal end 44 of theintroducer sheath 36.

Referring additionally to FIGS. 6 and 7, the working catheter 38comprises an elongated catheter body 60 having a proximal end 62 and adistal end 64. With respect to its composition, the catheter body 60 canbe generally divided into three sections: a bendable shaft section 66, adistal tip section 68, and an axially compressible section 70 disposedbetween the respective bendable and distal tip sections 66, 68. The term“axially compressible,” as used herein, means that the respectivecatheter section is more compressible than the adjoining bendable anddistal tip sections 66, 68. As such, when an axial force is placed onthe distal tip section 68 by the push-pull member 106 (or by an externalforce), the tip section 68 moves axially relative to the bendablesection 66, with the resulting compression or tension forces beingabsorbed by the axially compressible section 70, with the bendable andtip sections 66, 68 being relatively axially incompressible.

As best shown in FIG. 4, the bendable section 66 of the catheter body 60may be composed of a low-friction inner lubricious layer 72, astiffening layer 74 (e.g., a braided material or a metallic spine)disposed over the outer surface of the inner layer 72, and an outerpolymer jacket 76 disposed over the outer surface of the stiffeninglayer 74. Because the bendable section 66 is structurally reinforced byinclusion of the stiffening layer 74, torque transmission andinsertability of the catheter 38 is enhanced, while also providingenough cantilever bendability to facilitate access to remote tissuelocations, such as the chambers of the heart. As best shown in FIG. 5,the distal tip section 68 of the catheter body 60 is composed of theinner lubricious layer 72 and the polymer jacket layer 76, and is morelaterally flexible than the proximal section 66 due to the lack of astiffening layer.

Significantly, the axially compressible section 70 of the catheter body60, which in the illustrated embodiment in FIGS. 6 and 7 takes the formof a bellows, can axially elongate and contract in much the same way asan accordion, thereby allowing the distal tip section 68 to be axiallydisplaced relative to the bendable section 66 in response to theapplication of an axial (tension or compression) force exerted on thedistal tip section 68. As will be described in further detail below,this feature allows the distal end 64 of the catheter body 60 to beaxially dithered back and forth, while also facilitating the transfer ofaxial forces externally applied to the distal end 64 of the catheterbody 60 to the force sensing assembly 40.

As briefly discussed above, the working catheter 38 takes the form of anelectrophysiology/ablation catheter, and thus, comprises an ablationelectrode, and in particular, a tip electrode 78, and anelectrophysiology mapping electrode, and in particular, a ring electrode80 mounted around the distal end 64 of the catheter body 60 proximal tothe tip electrode 78. The electrodes 78, 80 may be composed of asuitably electrically conductive material, such as stainless steel orplatinum. The catheter may further comprise a temperature sensor 82(shown in phantom), such as a thermocouple or thermistor, suitablymounted within the tip electrode 78.

The working catheter 38 comprises electrical leads 84 extending througha wire lumen 86 (shown in FIGS. 4 and 5) within the catheter body 60,with the distal ends of the electrical leads 84 respectively terminatingat the tip electrode 78 and ring electrode 80, and the proximal ends ofthe electrical leads 84 terminating in the instrument driver 34(described in further detail below). The working catheter 38 alsocomprises an electrical lead 88 extending through a wire lumen 90 (shownin FIGS. 4 and 5) within the catheter body 60, with the distal end ofthe electrical lead 88 terminating at the temperature sensor 82, and theproximal end of the electrical lead 88 terminating in the instrumentdriver 34 (described in further detail below).

The tip electrode 78 optionally includes fluid irrigation ports 92through which a cooling fluid, such as saline, can flow into thepatient's body. In this case, the catheter 38 comprises a fluid supplytube 94 extending through the catheter body 60, with a distal end of thefluid supply tube 94 terminating within the tip electrode 78 in fluidcommunication with the irrigation ports 92, and a proximal end of thefluid supply tube 94 terminating at fluid inlet port at the proximal endof the working catheter 36 (described in further detail below).

The working catheter 38 further comprises a plurality of controlelements (in this case, four) in the form of steering wires 96 extendingthrough respective steering lumens 98 disposed within the wall of thecatheter body 60, and in particular, through the outer polymer jacketlayer 76 of the proximal section 66. The working catheter 38 furthercomprises a steering wire anchoring element in the form of an anchoringring 100, embedded within the outer polymer jacket layer 76 at thedistal end of the bendable section 66 of the catheter body 60. Therespective distal ends of the steering wires 96 (only one is shown inFIG. 7) are suitably mounted to the anchoring ring 100, and therespective proximal ends of the steering wires extend out the proximalend 62 of the catheter body 60, for being suitably coupled to instrumentdriver 34 (described in further detail below). In the illustratedembodiment, the proximal end 62 of the catheter body 60 includesapertures (not shown) through which the respective steering wires 96exit for coupling to the instrument driver 34. Thus, it can beappreciated that the distal end 64 of catheter body 60 can bealternately deflected in four different directions and straightened viaremotely controlled actuation of the steering wires 96.

The working catheter 38 has a central lumen 104 extending axiallythrough the catheter body 60 between the proximal and distal ends 62,64. The push-pull member 106 is slidably disposed within the centrallumen 104, wherein a distal end of the push-pull member 106 extendsthrough the anchor ring 100 and is affixed to the distal tip section 64(e.g., by soldering it to the inner surface of the tip electrode 78). Inone embodiment, the push-pull member 106 is a 0.010″ diameter 304SS wireattached to a solid metal electrode 78 forming the distal part of thedistal tip section 68 by soldering or laser welding. It will beappreciated that the push-pull member 106 should extend along theneutral (central) of the catheter body 60 to reduce and (if possible)eliminate the need for any geometric compensation.

A proximal end of the push-pull member 106 extends out from the proximalend 62 of the catheter body 60, so that it can be suitably coupled tothe force sensing assembly 40, as will be described in further detailbelow. In one embodiment, the push-pull member 106 may also function asa “safety wire” to protect the patient if the distal tip section 68 ofthe catheter body 60 should somehow separate from the compressiblesection 70. The push-pull member 106 may also be used as an electricallead 84 in lieu of one of the electrical leads 84 discussed above. Inthis case, the push-pull member 106 preferably includes an electricallyconductive core and an electrically insulative coating disposed over thecore.

The working catheter 38 optionally comprises a stiffening coil 108 thatalso extends axially through its central lumen 104, with the push-pullmember 106 extending through a center of, and being axially translatablerelative to, the stiffening coil 108. The stiffening coil 108 may becomposed of a material or be coated with a material that has a lowercoefficient of friction than that of the push-pull member 106. Forexample, the stiffening coil 108 may be coated withpolytetrafluoroethylene. In this manner, friction between the push-pullmember 106 and the inside surface of the stiffening coil 108 ispreferably minimized, as such friction may otherwise be excessive if toomuch surface area of the push-pull member 106 is in contact with theinside surface of the stiffening coil 108 when the bendable shaftsection 66 of the working catheter 38 is articulated in response toactuation of one of the steering wires 96. The coil 108 also facilitatesthe centering of the push-pull member 106 within the central lumen 104of the catheter 38.

In an alternate embodiment, depicted in relevant part in below-describedFIG. 15 a, a distal end portion 111 of the stiffening coil 108 mayextend past the steering wire anchor ring 100, and through the axiallycompressible section 70, before terminating at a proximal end surface 69of the distal tip section 68. In this distal end portion 111 of thestiffening coil 108, the pitch of the coil windings is significantlyopened and their diameters are substantially expanded, so that thedistal coil portion 111 functions as a resilient tensioning spring tohelp maintain the push-pull member 106 (the fluid supply tube 94′ inFIG. 15 a) in tension during the dithering operation.

Referring back to FIGS. 2 and 3, the instrument driver 34 providesrobotic steering actuation, as well as robotic insertion and retractionactuation, to the respective introducer sheath 36 and working catheter38 in accordance with control signals transmitted from the controlstation 16 (shown in FIG. 1). In particular, the instrument driver 34comprises a housing 110 that contains motors (not shown), an introducersheath interface to which the sheath splayer 112 is operatively mounted,and a working catheter interface to which the catheter splayer 114 isoperatively mounted.

The respective splayers 112, 114 are mechanically interfaced to thehousing 110 in such a manner that they may be axially displaced relativeto each other via operation of the motors, thereby effecting insertionor retraction movements of the respective introducer sheath 36 andworking catheter 38 relative to each other, and thus, relative to theoperating table 12 (shown in FIG. 1). Each of the splayers 112, 114comprises one or more rotating spools or drums 116 that can selectivelytension or release the steering wires 56, 96 disposed within therespective sheath body 42 and catheter body 60, thereby effectingdeflection of the distal ends 46, 64 of the sheath and catheter bodies42, 60. In some embodiments, the instrument driver 34, in conjunctionwith the support arm 14, may optionally be capable of rotating orrolling the sheath body 42 and catheter body 60 relative to each other.If the working catheter 38 alternatively or additionally includes anoperative element requiring mechanical actuation, the instrument drive34 and/or catheter splayer 114 may include additional spools (not shown)for tensioning control elements (not shown) used for actuating suchoperative element.

The proximal ends of the electrical wires 84, 88 exit from the proximalend 62 of the catheter body 60 into the catheter splayer 114, which thenexit the catheter splayer 114 as a bundle of wires that are terminatedin an electrical connector 118. A radio frequency (RF) generator andelectrophysiology mapping equipment (both not shown) can be coupled tothe electrical connector 118 to allow the transmission of RF energy andtemperature signals between the RF generator and the tip electrode 78and temperature sensor 82 (shown in FIGS. 6 and 7), and to allow thetransmission of signals between the ring electrode 80 (shown in FIGS. 6and 7) and the electrophysiology mapping equipment.

The proximal end of the fluid supply tube 94 exits from the proximal end62 of the catheter body 60 through the splayer 114, and terminates at aluer connector 120. A fluid pump (not shown) can be coupled to the luerconnector 120 to convey pressurized fluid into the fluid supply tube 94and out through the fluid delivery ports 92 on the tip electrode 78(shown in FIGS. 6 and 7). In an alternate “closed fluid cooling”embodiment (not shown), fluid from the fluid supply tube 94 circulateswithin an interior region of the distal tip section 64 and returns tothe proximal end of the catheter 38 through an additionally-providedfluid outlet tube. In such alternate embodiment, there are no fluiddelivery ports 92 in the tip electrode 78.

FIG. 14 depicts an alternate embodiment of the working catheter 38 thatmay be used in the robotic catheter assembly of FIG. 2, in which a soft,flexible polymer sleeve is used instead of the bellows for forming theaxially compressible section 70 of the catheter body 60. In particular,the compressible sleeve (which structurally coincides with thecompressible section 70, and is thus referred to hereinafter by the samereference number 70) has a proximal end joined by an adhesive weld orother known method to a distal end of the bendable section 66 at alocation just distal of the steering wire anchor ring 100. Similarly, adistal end of the compressible sleeve 70 is joined to the distal tipsection 68 at a location just proximal of a most proximal operativeelement 80.

The embodiment of FIG. 14 also includes a tensioning spring 81positioned axially within the compressible polymer sleeve 70 fortensioning the push-pull dithering member 106. In particular, a proximalend 83 of the spring 81 is compressible against a distal-facing surface101 of the steering element anchor ring 100, which also defines a distalend of the bendable section 66. A distal end 85 of the spring 81 iscompressible against a proximal-facing surface 69 of the distal tipsection 68. Prior to attachment of the push-pull member 106 to theditherer 122 (described below), the tensioning spring 81 is preferablyin a relaxed, unloaded state. When initiating the instrument system forperforming a procedure in a patient, the proximal end portion push pullmember is retracted proximally relative to the distal tip section 68 ofthe catheter body 60, thereby retracting the distal tip section 68relative to the bendable section 66, and compressing the spring 81against the distal anchor ring surface 101. Once a specified load (e.g.,approximately 80 grams in one embodiment) is reached on the push-pullmember 106 (as measured by the force sensor(s) 124, described below),the proximal end portion of the push-pull member 106 is affixed to theditherer 122, and dithering of the push-pull member 106 is commenced.The back-and-forth stroke distance traveled by the push-pull member 106is preferably governed by maintaining the measured (or “sensed”) load onthe push-pull member 106 within a specified operating range, (e.g., in arange of approximately 10-50 grams, in one embodiment).

As the ditherer 122 “pushes” the push-pull member 106 and, thus, thecatheter distal tip section 68 in a distal direction axially relative tothe instrument driver 34 on which the ditherer 122 is mounted, thetensioning spring 81 decompresses, decreasing the sensed load on thepush-pull member 106 until the lower end of the operating load range isreached, which is preferably (but not necessarily) at a point when thespring 81 returns to its unloaded state but without allowing the spring81 to being axially tensioned. Thereafter, the ditherer 122 reversesdirection and “pulls” the push-pull member 106 and, thus, the distal tipsection 68 in a proximal direction axially relative to the instrumentdriver 34, and the spring 81 is again almost compressed against theanchor ring surface 101, resisting the inward motion of the distal tipsurface 69, and increasing the measured load on the push-pull member 106until the upper end of the load range is reached. The dithering cycle isthen repeated.

It should be appreciated that the tensioning spring 81 is optional,although preferred, and that it may be used in other embodiments, suchas in the embodiment of FIGS. 6 and 7, in which the compressible sectioncomprises a bellows.

With any of the above-described embodiments, the proximal end portion ofthe push-pull member 106 exits from the proximal end 62 of the cathetersplayer 114 and terminates at the force sensing assembly 40 mounted onthe instrument driver 34. The force sensing assembly 40 generallycomprises a mechanical ditherer 122 and a force sensor 124. Althoughonly one force sensor 124 is shown and described herein, multiple forcesensors can be used in alternate embodiments. In the illustratedembodiment, the ditherer 122 is mounted to the housing 110 of theinstrument driver 34 proximal to the working catheter splayer 114. Theditherer 122 grasps the proximal end of the push-pull member 106 (e.g.,by providing a solder ball or the like on the proximal end of thepush-pull member 106), so that the push-pull members 106 can be axiallydithered back and forth by the ditherer 122, thereby axially ditheringthe distal end tip section 64 of the catheter body 60 back and forthrelative to the bendable section 66.

The length or stroke of the dithering may be adjusted depending on thenature of the procedure, but preferably is less than a few millimeters.In some embodiments, the proximal stroke of the dithering may be lessthan 1.5 mm. The frequency of the dithering may be several cycles persecond, e.g., 2-20 Hz, thereby ensuring that any static friction isbroken. It will be appreciated by those skilled in the art that some ofthe stroke is consumed by friction and compressibility, so the distalstroke is attenuated with respect to the proximal stroke. In someembodiments, the proximal stroke can be much reduced from 1.5 mm if thefrequency goes up to near 10 Hz, since the purpose of the dithering isto keep catheter in motion.

It will be appreciated that the force sensor 124 may be disposed atvarious locations along the push-pull member 106, including even at thedistal end 64 of the catheter body 60. In the illustrated embodiment,the force sensor 124 is disposed on the ditherer 122. The force sensor124 is used to detect the force or load that is being applied to thedistal end 64 of the catheter body 60 by detecting the force or loadthat is applied at the proximal end of the push-pull member 106. Thus,the force sensor 124 is able to sense the insertion and withdrawalforces applied to the distal tip 64 of the catheter body 60 via theditherer 122. In one embodiment, the proximal end portion of thepush-pull wire 106 has a crimp ball formed thereon at a proper distance,pre-set in relationship to the force sensor 124. By way of example, thecrimp ball may be over molded into a block or other feature that mateswith the force sensor 124.

One method for mounting the proximal portion of the push-pull member 106on the ditherer 122 is to start with the ditherer 122 in its completedforward-stroke, allowing a slight amount of slack in the push-pullmember 106. The ditherer would then be retracted distally until apredetermined load (e.g., 80 grams) is measured on the push-pull member106. A calibration routine also may be utilized to increase the accuracyof the sensed load.

The force sensing assembly 40 may optionally comprise a strain-gage (notshown) located at the distal end of the push-pull member 106 for sensinglateral or deflection forces applied to the distal end 64 of thecatheter body 60. The working catheter 38 may also comprise a sensor,e.g., an optical or capacitive sensor (not shown), located at the distalend 64 of the catheter body 60 to confirm that the distal end 64 isdithering back and forth.

Turning now to FIG. 8, one embodiment of a force sensing assembly 40used for measuring a force applied at the distal end 64 of the catheterbody 60 will be described. In this embodiment, the distal end 64 of thecatheter body 60 dithers with respect to the introducer sheath 36 andalso with respect to the more proximal bendable section 66 of theworking catheter 38. In order to axially dither the distal end 64 of thecatheter body 60 back and forth, the ditherer 122 drives the push-pullmember 106 through the force sensor 124, which measures the direct forceneeded to respectively insert and retract the push-pull member 106within the lumen 104 of the working catheter 38. The ditherer 122 ismechanically grounded (via a mechanical linkage 126) and is thus,stationary relative to the proximal end 62 of the catheter body 60. Solong as the introducer sheath 36 is not moved axially relative to theworking catheter 38, the ditherer is also stationary relative to theintroducer sheath 36. The force sensor 124 and push-pull member 106 movetogether relative to the proximal end 62 of the catheter body 60.

The force sensing assembly 40 is in operable communication with controlstation 16 via the communication link 24 for data processing. Inparticular, condition electronics 128 receives the electrical signalsgenerated by the force sensor 124, and the computer 28 processes theconditioned electrical signals. A representation of the axial forceapplied at the distal end 64 of the catheter body 60 can be displayed onthe monitor 26.

Over one or more dithering cycles, the force profiles or waveformsobtained from the force signals can be used to accurately estimate thecontact forces at the distal end 64 of the catheter body 60. Inparticular, the computer 28 obtains a baseline force measurement byreceiving signals from force sensor 124 when the push-pull member 106 isdithered back and forth at a time that no external axial force is beingapplied to the distal end 64 of the catheter body 60. The computer 28may then later obtain a total force measurement by receiving signalsfrom the force sensor 124 when the push-pull member 106 is dithered backand forth and an external axial force is applied to the distal end 64 ofthe catheter body 60 (e.g., when the distal tip of the catheter 38contacts tissue). The computer 28 then computes the external axial forceapplied to the distal end 64 of the catheter body 60 by subtracting thebaseline force measurement from the total force measurement. Notably,the total force measurement may capture signal induced by physiologicalcycles, such as the respiratory cycle and heart cycle. To interpret thissignal, such as for control purposes, the force sensing assembly 40 maycomprise a filter (not shown) for separating the physiologicalvariations within the total force measurement.

Further details on this type of force sensor system, along with variousother embodiments of dithering force sensor assemblies, are provided inU.S. patent application Ser. No. 11/678,001, which has previously beenincorporated herein by reference.

It will be appreciated that, as the working catheter 38 is articulated(bent) during a procedure, the catheter body 60 compresses. Additionallythere are geometric displacements on the catheter body 60, as well as onthe push-pull member 106 if it is located off the neutral axis of thecatheter shaft. Both of these displacements may be calculated andcompensated for by the varying the stroke length of the ditherer 122during operation. This compensation eliminates the forces generated bythe articulation of the catheter body 60 to be translated to the forcesensor 124.

With reference to FIGS. 16 and 17, the compensation mathematics involvedwith the mechanics of the working catheter 38 and push-pull member 106for one exemplary embodiment will now be described, including thevariables that contribute to the tension of the push-pull member 106when the catheter 38 is articulated. As seen in FIG. 16, the cathetermay have steering control elements or tendons 302, 304 and anarticulation section 306 and a non-articulating section 308 each withdifferent stiffness. The radius of curvature defining the amount of bendin the catheter is represented by r. FIG. 17 depicts a model of theforces applied to a catheter as it is articulated which treats thecatheter and each tendon member as springs with different springconstants depending on their stiffness. Since the articulation sectionvaries in stiffness from the proximal section, the axial stiffness ofthe catheter body 60 or stiffness in compression is represented as twodifferent spring constants k_(a) and k_(p). The catheter also hasstiffness in bending which is represented by the spring constant k_(b).Each of the tendons 302, 304 also has a stiffness k_(t2) and k_(t1)respectively. The tendons 302, 304 are shown offset from the center ofthe catheter by half the diameter of the catheter d/2.

To articulate the catheter, the tendons 302, 304 are displaced by adistance y₂ and y₁ respectively. In this model, a negative ydisplacement will place a tendon in tension T while a positive ydisplacement will place the tendon in slack or zero tension. When in astraight configuration, the tension in each tendon T_(y1) and T_(y2) areequal. In order to bend the catheter (as shown in FIG. 16), one tendon304 is placed tension T_(y1) in while the other tendon 302 is givenslack T_(y2)=0. The push pull member (shown as a displacement wire) 106is also shown running down the central axis and its displacement isshown as y_(dispwire).

The moments about the y and x axis will be 0 but in bending the momentsabout the z axis can be calculated as follows:ΣM=T _(y1)(d/2)=k _(b)(1/r)  (1)

Solving for T_(y1) we get:T _(y1)=2k _(b) /Dr  (2)

Summing the tension in the system using Hooke's law T=kx, we get:T _(dispwire) +T _(y1) =k _(a)(ΔL _(a) /L _(a))+k _(p)(ΔL _(p) /L_(p))+k _(t1)(ΔL _(t) /L _(t))  (3)

Plugging equation 2 into equation 3 for T_(y1) then solving forTdispwire gives:T _(dispwire)+2k _(b) /Dr=k _(a)(ΔL _(a) /L _(a))+k _(p)(ΔL _(p) /L_(p))+k _(t1)(ΔL _(t) /L _(t))  (4)T _(dispwire) =k _(a)(ΔL _(a) /L _(a))+k _(p)(ΔL _(p) /L _(p))+k_(t1)(ΔL _(t) /L _(t))+(−2k _(b) /Dr)  (5)

Equation 5 shows that, during articulation, the catheter body 60compresses by a ΔL for a given radius. In order for the tension of thepush-pull member 106 to remain constant during articulation, the member106 must also move by ΔL by the ditherer 122 (shown in FIG. 3). ΔL maybe determined through analytical and/or empirical methods. In equation 5above the known physical constants are the spring constants (ka, kp, kt,kb), the catheter diameter (D), and the catheter lengths (La, Lp, Lt).The radius of curvature r would be a user input. Thus, ΔL can bemeasured empirically for various points of r to create a table that canbe used to determine the dither stroke necessary to keep the tension inthe displacement wire constant during bending. The table could be usedto create a graph of displacement of the guide tendon vs. the overallstrain of the shaft (or change in length of the shaft) as shown in FIG.18. The graph can then be used analytically to determine ΔL with respectto tendon displacement.

Having described the robotic catheter system 10, an exemplary method ofusing the robotic catheter system 10 to perform therapeutic and/ordiagnostic functions on a patient will now be described. First, theintroducer sheath 36, with the working catheter 38 retracted therein, isintravascularly introduced through a puncture within the patient's bodyand robotically advanced/maneuvered through the vasculature of thepatient to a target site, such as a chamber of the heart, as illustratedin FIG. 9 a. In this case, the introducer sheath 36 is transeptallyintroduced into the left atrium of the heart. The working catheter 38 isthen robotically advanced out of the introducer sheath 36, as shown inFIG. 9 b. The distal end of the working catheter 38 is then axiallydithered back and forth relative to the proximal end of the workingcatheter 38, as shown by the arrows in FIG. 9 c. In the illustratedembodiment, this is accomplished by operating the ditherer 122 toaxially dither the push-pull member 106 back and forth, therebydithering the distal end of the working catheter 38 back and forth viaoperation of the bellows or alternatively designed compressible section70.

Preferably, as shown in FIG. 9 c, the distal end of the working catheter38 is axially dithered back and forth when an external axial force isnot presently applied between the tissue and the distal end of theworking catheter 38, and measuring the force at the force sensor 124 toobtain a baseline force measurement. While the distal end of the workingcatheter 38 is being dithered, it is robotically moved within at leastone-degree of freedom (e.g., by deflecting the distal end of the workingcatheter 38), thereby placing its distal tip section 64 in contact withtissue, as shown in FIG. 9 d. Because the force at the force sensor 124is continuously measured, the total force measurement will be obtainedas the distal end of the working catheter 38 is placed into contact withthe tissue. The axial force applied between the tissue and the distalend of the working catheter 38 can then be computed (in this case, bythe computer 28) by subtracting the baseline force measurement from thetotal force measurement.

The operative elements at the distal end of the working catheter 38 (inthis case, the tip ablation electrode 78 and mapping ring electrode 80)can then be operated to perform the therapeutic and/or diagnosticfunction (in this case, tissue ablation and/or mapping) on the patient.The distal end of the working catheter 38 can be moved to a differentregion on the tissue. The axial force applied between the tissue and thedistal end of the working catheter 38 can again then be measured and theoperative elements at the distal end of the working catheter 38 operatedto again perform the therapeutic and/or diagnostic function.

As briefly discussed above, the force sensing mechanism disclosed hereinand be used with medical devices with operative elements other thanablation/mapping electrodes. For example, operative elements, such asenergy delivering laser fibers, scalpel, grasper/tweezers; sensor(radiometer, IR, spectrometer (excitation light source in combinationwith a detector)), etc., can be used. For example, FIG. 10 illustrates arobotically controlled working catheter 150 having an elongated catheterbody 152, a pair of grasper arms 154 affixed to the distal end of thecatheter body 152, and a cable 156 extending through the catheter body152 and coupled to the grasper arms 154.

The grasper arms 154 can be spring-loaded to open relative to eachother, in which case, the cable 156 can be pulled to close the arms 154relative to each other. As with the working catheter 38, the workingcatheter 150 includes an axially flexible section 158, and a push-pullmember 160 slidably disposed within a central lumen 162 extendingthrough the catheter body 152. The distal end of the push-pull member160 is affixed to the catheter body 152 at a point distal to the axiallyflexible section 158, and in particular, to the distal end of thecatheter body 150, and a proximal end that extends out from the proximalend of the catheter body 152, so that it can be suitably coupled to theforce sensing assembly 40 in the manner discussed above. As with thecatheter 38, the catheter 150 optionally comprises a centering coil 162affixed around the push-pull member 160.

The force sensing mechanism disclosed herein can be used to performmedical procedures in anatomical regions other than the heart. Forexample, as shown in FIG. 11, the working catheter 150 can be used toperform a surgical procedure within the cavity of the stomach 170 whilesensing the force between the grasper arms 154 and the wall of thestomach to prevent perforation of the inner lining of the stomach.

It will be appreciated that there must be adequate slack in the fluidsupply tube 94 to allow for the distal tip section 64 to be ditheredrelative to the bendable section 66 without tensioning and/orcompressing the fluid supply tube 94. Further, the fluid supply tube 94must be sufficiently flexible, and preferably have a low friction, e.g.,coated, exterior surface in order to minimize internal frictional forceswithin the catheter lumen 104 created by its presence. This isparticularly important since the fluid supply tube 94 is “dithered”along with the distal tip section 64 to which it is attached relative tothe rest of the catheter body 60. For example, the fluid supply tube 94may be provided with a service loop (not shown) or its own bellows (notshown) within the interior of axially compressible section 70 of thecatheter body 60. However, such features necessarily take up space andmay be difficult to implement without interfering with the push-pullmember 106. Such features may also result in an irregular amount offluid being delivered out the ports 92 and/or add undesirable stiffnessto the axially compressible section 70.

In yet another alternate embodiment of the working catheter 38, shown inFIGS. 15 a and 15 b, these issues are resolved by using the coolingfluid supply tube (designated 94′) as the push-pull member for ditheringthe distal end section 68 relative to the bendable section 66. Inparticular, the fluid supply tube 94′ is extended down the center of theworking catheter central lumen 104, and (as depicted in FIG. 15 a) downthe axial lumen of an (optional) stiffening coil 108, through thesteering wire anchor ring 100, and then through the compressible section(e.g., a flexible polymer sleeve) 70 of the catheter body 60 to thedistal tip section 68.

As mentioned above, a distal end portion 111 of the stiffening coil 108extends through the open center of the steering wire anchor ring 100,and through the axially compressible section 70, before terminatingagainst a proximal end surface 69 of the distal tip section 68. Inparticular, in the distal portion 111 of the stiffening coil 108, thepitch of the individual coil windings is significantly opened, and thediameter of the individual windings is substantially expanded, so thatthe coil portion 111 functions as a resilient tensioning spring to helpmaintain the fluid supply tube 94′ in tension during the ditheringoperation, much in the same manner as the tensioning spring 81 maintainsthe push-pull member 106 in tension in the above-described embodiment ofFIG. 14.

As seen in FIG. 15 b, a proximal end portion 138 of the fluid supplytube 94′ extends out the proximal end of the working catheter 38 and maybe mechanically coupled (e.g., grasped by) the ditherer 122 inessentially the same manner as the push-pull member 106 inabove-described embodiments, except that some accommodation may beneeded in order to not “pinch” the fluid supply tube 94′ in a mannerthat restricts fluid flow there through. A service loop 140 is providedin a still more proximal portion 142 of the fluid supply tube 94′extending proximally of the ditherer 122 to provide slack needed for thefluid supply tube 94′ to be used as the push-pull dithering member. Thefluid supply tube 94′ is otherwise connected to a fluid source in asimilar manner as fluid supply tube 94 in the previously describedembodiments. As with the tensioning spring 81 in the embodiment of FIG.14, the distal portion 111 of the stiffening spring 108 is preferably inan unloaded state prior to attachment of the fluid delivery tube 94′ tothe mechanical ditherer 122.

Thus, the working catheter 38′ of FIGS. 15 a and 15 b provides for axialdithering of the distal tip section 64 relative to the bendable section66, while still providing a fluid supply for open irrigation in thebody. The fluid delivery tube 94′ is preferably stiff, just like thepush-pull wire 106 of the previously described embodiments, and is alsopreferably kept in tension by the distal portion 111 of the stiffeningcoil 108. Advantageously, no stress relief bellows or service loop isneeded along the distal portion of the fluid supply tube 94′ to allowfor the dithering operation.

Although the previous embodiments of the working catheter 38 haveaxially compressible section, such as a compressible polymer sleeve or abellows, with or without an additional spring member for tensioning thepush-pull member, in order that the distal tip section can be axiallydisplaced relative to the remaining portion of the catheter body, an“axially translatable” section can be used in alternate embodiments toperform this function.

For example, referring to FIG. 12, a working catheter 200 is similar tothe above-described working catheter 38′, with the exception thatinstead of an axially compressible section 70 (e.g., a compressiblepolymer sleeve or bellows), the working catheter 200 includes an axiallytranslatable section (designated 70′) that has a seal 202 suitablymounted to the distal end of the bendable shaft section 66. The seal 202includes an annular aperture 204 through which the distal tip section 68is disposed. The diameter of the aperture 204 is equal to or less thanthe outer diameter of the distal tip section 68. Thus, the distal tipsection 68 is slidably disposed within the more proximal bendable shaftsection 66 via the seal 202, which prevents bodily fluids, such asblood, from entering the working catheter 200 via the interface betweenthe respective catheter shaft sections 66, 68. The distal end of thepush-pull member 106 is affixed to the distal tip section 68, and thus,is displaced with the distal tip section 68 in the manner and with theresult described above with respect to the working catheter 38.

The seal 200 may be composed of a suitable material, such as rubber, toallow the respective bendable and distal tip sections 66, 68 to easilyslide relative to the each other while maintaining a good sealtherebetween. The proximal end of the distal tip section 68 preferablyincludes an annular flange 206 that abuts the seal 202 during thefarthest extent of distal tip section 68, thereby preventing the distaltip section 68 from disengaging from the bendable shaft section 66. Inalternative embodiments, the seal 202 is suitably mounted to theproximal end of the distal tip section 68, in which case, the bendableshaft section 66 will be slidably disposed within the distal tip section68 via the seal 202.

In another alternative embodiment illustrated in FIG. 13, the workingcatheter 200 includes a pair of seals 202—one seal 202(1) mounted to thedistal end of the bendable shaft section 66, and another seal 202(2)mounted within the bendable shaft section 66 proximal to the first seal202(1). Each of the seals 202 includes an annular aperture 204 throughwhich the distal tip section 68 is disposed. Thus, the seals 202 providesuitable bearing surfaces that maintain axial alignment of the distaltip section 68 within the bendable shaft section 66. In this embodiment,the distal end of a fluid tube 94 extending through the catheter body 60terminates in an aperture 208 within the proximal seal 202(2). Thus,fluid can be conveyed into a chamber 210 formed between the seals 202,thereby creating a positive pressure therein. As a result, any bodilyfluids, such as blood, that would otherwise leak through the distal seal202(1) is prevented from entering the working catheter 200 due to thepositive pressure. Furthermore, apertures 212 are formed in the distaltip section 68 through which the positively pressurized fluid can beconveyed to the irrigation ports 92 on the electrode 78 to provide theirrigation function described above.

While multiple embodiments and variations of the many aspects of theinvention have been disclosed and described herein, such disclosure isprovided for purposes of illustration only. Many combinations andpermutations of the disclosed system are useful in minimally invasivesurgery, and the system is configured to be flexible. Where methods andsteps described above indicate certain events occurring in certainorder, those of ordinary skill in the art having the benefit of thisdisclosure would recognize that the ordering of certain steps may bemodified and that such modifications are in accordance with thevariations of the invention. Additionally, certain of the steps may beperformed concurrently in a parallel process when possible, as well asperformed sequentially as described above. Thus, it should be understoodthat the invention generally, as well as the specific embodimentsdescribed herein, are not limited to the particular forms or methodsdisclosed, but also cover all modifications, equivalents andalternatives falling within the scope of the appended claims.

1. A method for sensing force on a robotically controlled medicalinstrument, the method comprising: introducing a medical probe into apatient, the medical probe having an elongated probe body comprising acentral lumen, and a push-pull member extending axially through thecentral lumen, a distal end of the push-pull member being attached to adistal end portion of the probe body; mechanically displacing a proximalend of the push-pull member back and forth in a cyclical manner withoutany external force applied to the distal end portion of the probe body;mechanically displacing the proximal end of the push-pull member backand forth in a cyclical manner with external force axially appliedbetween internal body tissue of the patient and the distal end portionof the probe body; and determining the external force applied betweenthe internal body tissue of the patient and the distal end portion ofthe probe body, wherein the external force applied between the internalbody tissue of the patient and the distal end portion of the probe bodyis determined by obtaining a baseline force measurement when the distalend portion of the probe body is axially dithered back and forth withoutany external axial force applied to the distal end portion of the probebody, obtaining a total force measurement when the distal end portion ofthe probe body is dithered back and forth with the external axial forceaxially applied between the tissue and the distal end portion of theprobe body, and subtracting the baseline force measurement from thetotal force measurement.
 2. The medical method of claim 1, the probebody further comprising an axially variable-length portion interposedbetween the respective distal end and more proximal portions of theprobe body.
 3. The medical method of claim 2, wherein the push-pullmember comprises a fluid delivery tube having an open distal end coupledto the distal end portion of the probe body.
 4. The medical method ofclaim 2, wherein the axially variable-length portion of the probe bodycomprises a spring.
 5. The medical method of claim 2, wherein the probebody is robotically controlled by an instrument driver having an adapterconfigured to be operatively coupled to one or more steering wiresextending through the probe body for controllable bending of a bendableportion of the probe body in at least one direction, the one or moresteering wires having distal ends affixed to the probe body at asteering wire anchor member located at a distal end of the bendableportion, the push-pull member extending distally past the anchor memberto the distal end portion of the probe body.